Tony was kind enough to lend that article to IceInSpace too, Doug
Avoid Histogram Clipping in ImagesPlus
Leon - Paul explained it well enough. But to add to it, if you raise the black point, you darken your background which is often what you want. But if you raise it too far, you'll start removing the real detail (the faint stuff) as well. Similarly from the other end of the histogram, raising the white point will bring the levels up and make faint objects brighter - but you might start clipping already bright objects and making them pure white.
Ideally your histogram should spread across the whole range - that you'd call a broad dynamic range. If your histogram is a small peak that you squash the black and white levels in-between, you're reducing your dynamic range.